Asteroid 2020 YS2 passed by the Earth at a distance of about 87 000 km (0.23 times the average distance between the Earth and the Moon, or 0.06% of the distance between the Earth and the Sun, or slightly more than 2.5 times the distance at which satellites in geostationary orbits circle the Earth), slightly after 6.00 am GMT on Sunday 20 December 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2020 YS2 has an estimated equivalent diameter of 1-4 m (i.e. it is estimated that a spherical object with the same volume would be 1-4 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) more than 42 km above the ground, with only fragmentary material reaching the Earth's surface.
2020 YS2 was discovered on 21 December 2020 (the day after its closest approach to the Earth), by he 0.5-m Asteroid Terrestrial-Impact Last Alert System telescope on Mauna Loa in Hawaii. The designation 2020 YS2 implies that it was the 68th asteroid (asteroid S2 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., so that S2 = (24 x 2) + 18 = 68) discovered in the second half of December 2020 (period 2020 Y - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
2020 YS2 has a 354 day (0.95 year) orbital period, with an elliptical orbit tilted at an angle of 2.58° to the plain of the Solar System which takes in to 0.89 AU from the Sun (89% of the distance at which the Earth orbits the Sun) and out to 1.03 AU (103% of the distance at which the Earth orbits the Sun). Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2020 YS2 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. Close encounters between 2020 YS2 and Earth are fairly common, with the last thought to have happened in December 2019 and the next predicted in September 2027.
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